This section introduces various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion will assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. This section should be read in this light, and not necessarily as admissions of prior art. In the following specification, the invention is described in the context of strain-based design of pipelines. However, the invention is clearly of wider application to any situation in which a high strength, high toughness weldment is desirable, including but not limited to any non-pipe weldments of any one or more steel materials. Various terms are defined in the following specification. For convenience, a Glossary of terms is provided immediately preceding the claims.
With respect to applied loads, design standards, and material performance requirements, traditional pipelines are designed to prevent the pipeline materials from experiencing significant plastic strains. This type of design is referred to as allowable stress design or stress-based design. In stress-based designs, the loads applied to the materials are typically limited to some fraction of the yield strength of the construction. While in some cases, local plasticity might occur in a stress-based-designed pipeline at small stress concentrations like weld toes (i.e., over dimensions of several millimeters), generally stress-based designs are not intended for situations where large areas (many inches or feet) of the pipeline are subjected to plastic strains.
Today, pipelines are being designed for increasingly hostile service environments. For some demanding environments such as discontinuous permafrost, seismic, iceberg scouring, etc. where service temperatures can range as low as −20° C. or lower, there is a need to design and build pipelines capable of withstanding some degree of plastic deformation. In such cases, the deformation is largely oriented parallel to the pipe axis (i.e., longitudinal plastic strains) and the applied loads are often described in terms of applied global strains which are experienced over many inches or possibly feet of pipeline material. Strain-based design (SBD) is the term used to describe designing/constructing a pipeline that is capable of incurring longitudinal plastic strains. Typical strain magnitudes for strain-based designs are generally defined as global plastic strains in excess of 0.05%. Global plastic strains are defined as strains measured along a length of pipe and straddling the weld or welds in question that are not local but are spread over a distance of many inches or feet. In the case of an oil or gas pipeline, for example, global plastic strains for strain-based design purposes could be in reference to a section of the pipeline that is about two pipe diameters in length, although other similar definitions could be used to define global plastic strains. Using this convention, a global plastic strain of one percent in a 30 inch diameter pipeline would produce about 0.6 inches of strain in two diameters of length; i.e., 60 inches in length.
Fracture mechanics techniques called engineering critical assessment (ECA) are used to judge the structural significance of defects in girth welds for stress-based design pipelines. ECA includes accepted practices for testing materials, qualifying welds, and assessing the significance of weld imperfections in stress-based designs. Strain-based design (SBD) is not as mature a field as traditional stress-based design, and as of 2010, fully validated ECA practices for SBD have not been widely accepted by the pipeline industry. However, ECA principles are applicable to SBD. Many aspects of SBD pipeline engineering have been published at recent international conferences. Several notable venues include the Conference of Pipeline Technology in Belgium, the International Pipeline Conference in Canada, and the annual conferences of The International Society of Offshore and Polar Engineers (ISOPE) and The Offshore Mechanics and Arctic Engineering Society (OMAE). ExxonMobil has published numerous articles at these conferences including topics such as prediction methods for girth weld defect tolerance under SBD loading conditions, full-scale pipe testing for SBD engineering, fracture mechanics test methods, and girth welding technology useful in SBD applications. These publications in combination with patent applications International Application Numbers PCT/US2008/001753 (WIPO Patent Application WO/2008/115323, A Framework To Determine The Capacity Of A Structure) and PCT/US2008/001676, (WIPO Patent Application WO/2008/115320, Method To Measure Tearing Resistance) provide the background necessary for strain-based design engineering critical assessment (SBECA) technology to one skilled in the art.
Depending on the service temperature and applied loads, common structural steels and welds can experience either brittle or ductile fracture. Ductile fracture occurs at higher temperatures and brittle (or “cleavage”) fracture occurs at lower temperatures. At some intermediate temperature range, a transition occurs between ductile and brittle fracture. This transition is sometimes characterized by a single temperature called the ductile-to-brittle transition temperature (DBTT). The DBTT can be determined by use of the Charpy V-notch or CTOD test, depending on the application.
In stress-based design applications materials engineering and pipeline design practices are focused on ensuring adequate brittle fracture resistance and little attention is paid to ductile fracture of the girth welds. Brittle fracture is mitigated by specifying a minimum design temperature (consistent with the lowest anticipated service temperature) and using test methods like the Charpy V-notch or crack tip opening displacement (CTOD) test to qualify materials.
In the newer application of SBD pipelines, however, it is necessary to go beyond the simple consideration of brittle fracture; ductile fracture of the girth welds must also be considered. Girth welds are usually considered potentially the weakest link due to the common presence of degraded microstructures and imperfections caused by welding. In strain-based design, the designer, through choice of materials, welding, and inspection technology, will mitigate brittle fracture, or at least to delay it until well into the plastic loading regime and beyond the designed strain demand. During plastic loading of a pipeline, ductile tearing can initiate at girth weld defects. Depending on such factors as the strength properties and ductile tearing resistance of the welds, defect size, and pipeline base steel, the amount of tearing can be minimal and stable. If stable, the amount of defect growth typically ranges from a few microns up to a millimeter or two. If this degree of growth can be reliably accounted for in strain-based pipeline engineering practices, and specifically SBECA procedures, then pipeline integrity can be quantified and managed. For these reasons, overmatched girth welds with good ductile tearing resistance are important for SBD pipelines.
Naturally, there is an inherent tradeoff between strength and toughness in structural steels and weldments. As strength increases, toughness generally decreases. SBD requires both higher strength and higher toughness. A primary challenge for SBD pipelines is how to obtain both high strength and high toughness in the girth welds. The properties of pipeline girth welds are primarily controlled by the microstructure, which is in turn controlled by the chemistry and thermal cycle imposed during welding. Chemistry is mostly controlled by the base material of the pipe and selection of the welding consumables (wire, shielding gas, and/or fluxes). The weld thermal cycle is primarily a product of the weld procedure and base material thickness.
One potential consideration to obtain adequate toughness is use of highly alloyed (e.g., Ni-base alloys) welding filler wires because increasing nickel content generally creates toughness improvement. This approach has been used in cryogenic applications such as the welding of 9% Ni steel. There are two problems with this approach. The first is that the weld metals in such welds have an austenitic microstructure and are inherently weak. When welding 9% Ni steel, the austenitic welds are notably weaker than the base metal and the designs are typically de-rated according to the strength achieved by the Ni-base welds rather than the full strength of the 9% Ni steel. Although these Ni-base wires are not currently used for oil and gas pipelines, if they were considered for use due to their good toughness properties, they would only generate strengths useful for pipe grades up to about X60. Secondly, Ni-base filler wires are problematic for welding structural steels because high Ni weld metals are viscous when molten and difficult to weld. Once nickel content exceeds about five wt %, the viscosity of the weld metal can be noticeably higher. The poor fluidity of the viscous weld metal increases the chance of creating weld defects. This is particularly problematic for mechanized 5G pipeline girth welds where the constantly changing weld position and tight bevels creates a challenging situation that demands a good wetting, smooth operating, welding method.
U.S. Pat. Nos. 3,218,432 and 3,902,039 describe the above approach to achieve higher strength cryogenic welds as compared to typical austenitic (Ni-base) welds. These patents disclose ferritic filler wires comprising about 9-12 wt % Ni. These ferritic wires will hereafter be referred to as Fe—Ni wires and the associated welds, Fe—Ni welds. When used to weld 9% nickel steel the welds are stronger than welds in 9% Ni steels made using Ni-based alloys. Filler metals based on the teachings of these two patents have been commercialized, however they are rarely used. In order to achieve good cryogenic toughness, the Fe—Ni welds must be made using the gas tungsten arc welding (GTAW) process with low heat inputs and strict welding controls must be maintained, and these welds are difficult to make out of position because of the viscous weld metal. Controls are necessary to minimize weld metal oxygen content (related to toughness) and weld defects such as porosity, hot cracking, and lack of fusion. Many fabricators of cryogenic structures are unwilling to operate under these restrictions, believing that reliability may be unmanageable and productivity will be poor. While the use of Fe—Ni welds can theoretically achieve toughnesses suitable for cryogenic applications, Ni-based austenitic wires continue to be the most commonly used welding consumable for cryogenic applications despite the drawback of low strength.
An important difference to remember between cryogenic welding and the present invention is that cryogenic welded designs are distinctly stress-based and the materials are designed to prevent brittle fracture. Cryogenic designs operate at stresses well below the yield strength of the engineering alloys. Ductile fracture and tearing resistance are not a consideration for cryogenic design and the aforementioned Fe—Ni welding techniques were not purposefully designed to produce good ductile fracture resistance.
One approach to producing steel pipe welds that are useful for strain-based design is disclosed in U.S. Patent Application Publication No. US PA 2010/0089463, published Apr. 15, 2010 (International Patent Application PCT/US2008/001409) which discloses the use of austenitic filler wires to weld pipe for strain-based pipeline designs. The publication teaches the production of high toughness welds using Ni-based alloy, stainless steel, or duplex stainless steel welding consumables. This invention is hereafter called the “austenitic SBD weld”. This publication teaches away from ferritic weld metals in that it states conventional ferritic welds have limitations in toughness and tearing resistance that restrict the amount of strain that can be accommodated in structural design. A weld that achieves toughness suitable for SBD applications, but is significantly stronger than the austenitic SBD weld is disclosed in this application below.
When austenitic welds are applied to ferritic steels, a dissimilar atomic structure weld interface is created at the boundary between the weld metal and the weld heat affected zone (HAZ). Austenite possesses a face centered cubic (fcc) structure and ferrite possesses a body centered cubic (bcc) structure. Application of ultrasonic testing/inspection to dissimilar interfaces for defects such as lack of fusion can be difficult because this interface produces sound reflections that can be misinterpreted. Fcc and bcc materials have different sound propagation properties and respond differently to ultrasonic inspection. For challenging applications like SBD, it is desired to inspect for small defects with a tolerance on the sizing accuracy on the order of a millimeter. Dissimilar weld interfaces can cause signals during UT inspection that rival the signals created by small defects or at least create uncertainties in sizing accuracy. This is particularly the case for signals that emerge from a dissimilar weld in an area of the heat affected zone that has other geometric complexities like cusps or scallops between adjacent weld beads or in areas where the weld bevel geometry has changed. For the above reasons, it is desirable that ferritic steel pipelines be joined with ferritic welds to avoid dissimilar weld interface and enable accurate inspection when using UT inspection.
There is a need for weld metal that simultaneously produces high strength, high ductile fracture resistance, and good brittle fracture resistance (i.e., good ductile and brittle fracture toughness) and that can be applied during pipeline field construction without undue concern regarding “weldability” or ease of use in terms of weld pool control and defect rates.
The present invention comprises a ferritic weld produced by a ferritic welding consumable that achieves high toughness and tearing resistance, even at low temperatures. The invention provides exceptional strain hardening capacity, excellent defect tolerance, and high strain capacity in SBD applications, and provides excellent weldability, high strength, and accurate ultrasonic inspection.